The development of wireless communication networks making use of the 5 GHz frequency bands was initially restrained because these bands were originally reserved for use by radar systems. Recently, the coexistence between radars and communication systems in a 5 GHz frequency bands has been studied and regulatory offices have allowed systems such as Wireless Local Area Networks (WLANs) to use these bands under strict rules to be conformed with.
More precisely, equipment operating in the 5 GHz frequency bands may have dynamic frequency selection (DFS) capability. Dynamic frequency selection (DFS) is the mechanism that has been devised to enable operation of wireless equipments, for example wireless local area networks (WLANs) based on the HIPERLAN2 and IEEE802.11a/b/g/h standards (which are incorporated by reference), in the 5 GHz frequency bands without causing harmful interference to radar systems operating in those bands. The main principle of DFS is that devices operating in the 5 GHz frequency bands shall be capable of detecting radar signals and shall avoid the channels used by radars.
The principle of DFS is described in the International Telecommunication Union (ITU) recommendation (ITU-R M.1652) and the regulatory requirements for Europe can be found in the Conference of European Postal and Telecommunications (CEPT) decision ERC/DEC/(04)08 (all of which are incorporated by reference).
In order to comply with regulatory requirements, systems using license-exempt bands, such as IEEE 802.11 wireless local area networks and IEEE 802.16 wireless access networks, have introduced messages in the Media Access Control (MAC) protocol enabling operation of DFS. However, the mechanism to perform the detection of radar and of other communication systems is not defined by any of these standards and is left to the system manufacturer.
Though not all of them are explicitly stated, the main requirements for the DFS mechanism are:
The DFS and radar detector shall be capable to quickly detect radar, an order of magnitude being 0.2 s, above a given power level with a very low non-detection probability.
The DFS and radar detector shall be performed both during silent periods of the 5 GHz wireless systems and also during their active periods.
The DFS and radar detector shall be very reliable in the sense that the probability of false detection, such as detecting a radar if no radar is actually present, shall be negligible, because a channel on which a radar has been detected shall no longer be used for 30 minutes.
The DFS and radar detector shall be capable to detect the use of a channel by another wireless transmission system, for example Wireless Local Area Network (WLAN) or Wireless Metropolitan Area Network (WMAN) system.
Dynamic frequency selection (DFS) mechanisms and conformance tests for coexistence in the 5 GHz frequency bands have been standardized and published only very recently. As a consequence, little information on the subject is available in the open literature.
The most relevant solutions in the state-of-the-art are described in the applications US 2003/0107512 concerning a “radar detection and dynamic frequency selection for wireless local area networks”, and in the application US 2004/0033789 concerning a “dynamic frequency selection and radar detection with a wireless LAN”. Both of these applications are incorporated by reference.
The application US 2003/0107512 presents an exhaustive study of the problems raised by DFS and radar detection. However, it does not propose any practical solution to do it, and it only lists all the possible tools that a designer can rely on to design radar detector. All these tools are very classical and known by any skilled in the art person working in the field, for example time domain analysis, spectral analysis, Fast Fourier Transform. Besides, it never really explained clearly how to detect radar in practice.
The application US 2004/0033789 presents a mechanism based on a video detector and a complicated analysis. This implementation is hardly possible in practice and cannot be used in an embedded system.
As a consequence, the existing methods fall short of fulfilling the requirements of a practical Dynamic Frequency Selection (DFS) solution, and no existing method seems to be practically implemented and straightforwardly put in practice.